1. Field of the Invention
This invention relates to an information signal selecting apparatus for transmitting an electrical video signal from a plurality of transmitting means to each of a plurality of receiving means and for selectively receiving the transmitted video signal by each receiving means.
2. Description of the Prior Art
In a monitoring television system used as an information transmission system, at least one television monitor and one video recorder are connected through an electronic matrix switch to a plurality of television cameras and a plurality of transmission lines, for selectively connecting at least one camera for display through the monitor and record another camera signal onto the video recorder. The electronic matrix switch is used also for sequentially connecting a plurality of the television cameras to a monitor and independently recording other camera signals in sequence onto the video recorder. Such electronic matrix switch uses a centralized controller for the selection of all the outputs, be it a single television camera or sequencing multiple television cameras, for displaying on the monitor or recording independently any single or multiple camera signals in sequence onto the video recorder.
However, in a well known electronic matrix switching used for a system of this kind, as the number of television cameras and monitors or video recorders increase, the centralized switching and coordination become more complex, requiring larger software and hardware capacity to control the multiple individual independent outputs. Thereby, as the system grows, the matrix switching control becomes costly and inefficient.
Further, well known electronic matrix switches do not employ synchronized switching and the selected images displayed on a monitor are often disturbed during and immediately after the switching-over from one television camera to another.
In such an information switching system, it is preferable to mutually lock the internal synchronizing signals of a plurality of television cameras and the electronic switching time to an external synchronizing signal, in order to prevent the picture image on the monitor from being disturbed during and immediately after the switching operation from one television camera to another.
As the one of the devices for synchronizing a plurality of television cameras, there is known an apparatus for transmitting a vertical synchronizing signal and a horizontal synchronizing signal or composite synchronizing signal used in a television system. Another known apparatus is an apparatus for transmitting a vertical drive signal and a horizontal drive signal, and driving the television camera and its deflection circuits by the transmitted vertical drive signal, and the horizontal drive signal.
In any of the above known devices of the type under discussion, as the transmitted synchronizing signal itself is a train of pulses, which can be easily influenced by noise, the transmission of a synchronizing signal requires the use of a coaxial cable with its high shielding effect, which makes it costly for systems with a plurality of television cameras.
Another known apparatus for synchronizing a plurality of television cameras, is an apparatus for transmitting an external synchronizing signal from an external synchronizing generator to the television cameras by injecting the external synchronizing signal into the video signal transmission line and locking an internal synchronizing signal generator of the television camera by means of the transmitted external synchronizing signal. Such an apparatus is disclosed in U.S. Pat. No. 4,603,352 the contents of which are incorporated herein by reference.